It is nonsense to say the US will default on their debts. There is plenty of money available coming in each month in taxes to pay the interest. As long as the interest is paid there is no default.
This is true, mostly. The U.S. would not "default" on existing debts so long as they pay the interest. However, there are spending bills that have already passed Congress and have ongoing expenses. If the debt ceiling is not raised the U.S. cannot finance those already incurred obligations. So if you think it is a good idea to withhold, say, veteran's benefits (along with hundreds of other programs), then go ahead and don't approve the debt ceiling extension.